The present invention relates generally to Precision Approach Path Indicator (PAPI) visual guidance systems for aiding pilots in landing an aircraft. Specifically, the PAPI system defines the vertical approach angle to the runway and indicates to the pilot, via colored lights, whether the angle of approach of the aircraft to the runway is correct. The colored lights are produced in a number of Lamp Housing Assemblies (LHAs), as will be described below. The Federal Aviation Administration (FAA) establishes the standards for PAPI systems in the United States, whereas the standards for foreign PAPI systems may differ. It should be understood that, while the present invention is described with respect to the FAA endorsed systems, its application isn't limited to FAA endorsed systems.
The components in an FAA Style B PAPI system are powered by the well-known and widely used constant alternating current (AC) loop used in most of the world's airport lighting systems, whereas the components in an FAA Style A PAPI system are powered in parallel directly from utility line power. In any PAPI system their are a number of important considerations, among them being: power consumption; number and type of lamps; size of the LHA; system reliability; ease of installation and service; safety with respect to exposed wiring and high voltages; ease of detection and identification of lamp or housing problems; environmental impact of components used; and minimization of the number of wires and interconnections.
The PAPI system generally comprises an array of two or four Lamp Housing Assemblies (LHAs), each of which may contain two or three individual lamps. The LHAs are located adjacent the side of, and perpendicular to, a runway and precisely aimed to define a correct vertical approach angle for guiding an incoming aircraft. Each LHA is usually fitted with an optical filter to present a white light when the aircraft is too high, i.e., above the correct approach angle, and a red light when the aircraft is too low or below the correct approach angle. When the aircraft is too high, all of the LHAs will be seen as white lights, when the aircraft is too low, all of the LHAs will be seen as red lights and when the aircraft is within the correct approach angle, one-half of the LHAs in the array will present a white light and one-half will present a red light. Generally the PAPI system comprises either two LHAs or four LHAs, with each LHA having either a set of two lamps or a set of three lamps. The two or three lamp sets appear as a single light when viewed at a far distance. A two LHA system will therefore show: two sets of red lights when the aircraft is too low; one set of white lights and one set of red lights for a correct approach; and two sets of white lights when the aircraft is too high. A four LHA system will also indicate intermediate positions within the correct approach angle. Thus the light indications will be: four sets of red for too low; one set of white and three sets of red for slightly low; two sets of white and two sets of red for correct approach angle; three sets of white and one set of red for slightly high; and four sets of white for too high. Each LHA also includes a tilt detection system and tilt switch control circuitry for disabling the entire LHA array should the physical attitude or positioning of any of the LHAs be disturbed a predetermined amount. This is necessary since the color of the light seen by the pilot could be erroneous and create a potentially hazardous situation should the LHA position be disturbed sufficiently to change its aiming. The choice of PAPI system selected is determined by a number of factors, such as airport size, aircraft size and location, traffic density, economics and the like. For example, some airport installations use multiple PAPI systems located at differing distances (touch down points) along the runway to accommodate aircraft having different landing requirements. The present invention is useful in all PAPI Style B systems.
The current state-of-the-art FAA Style B PAPIs include one or more series isolation transformers for the lamps in each LHA and a separate transformer at a designated master LHA to power tilt switch circuitry. A master LHA also contains the control and timing circuits for the tilt system, which includes a tilt status monitoring loop and a tilt control loop. Each LHA has a tilt switch and some mechanism to cause the master LHA to disable the entire LHA array should a tilt condition occur in any of the LHAs. In practice a tilt status signal from each LHA is sent to the master LHA which, in turn, directs each of the LHAs in the array to disable its light output, via a shorting device such as a relay, should a tilt condition occur at any of the LHAs. In order to maintain the constant current series circuit, it is also common practice to have either; (a) one isolation transformer per lamp in each LHA or; (b) a single transformer and a lamp bypass circuit for each lamp in the LHA. Therefore, with each LHA having either two or three lamps, two or three isolation transformers are required, or one isolation transformer and two or three lamp bypass circuits are required. Such systems are relatively costly and consume a substantial amount of electrical power.
The LHA is mounted above ground adjacent to the runway and connected to a small container, colloquially referred to as a “handhole” that is buried behind the LHA. Each container includes a current transformer and is connected to a main constant current source and to the other hand-holes via an underground conduit or directly buried cabling. It will be appreciated that a minimum number of wires and connection points in the PAPI system is a desirable objective with respect to cost, installation and maintenance. Also, it is desirable to minimize the amount of above-ground equipment to avoid damage to or from vehicles and aircraft. In general, the handholes and LHAs are interconnected through break-away type connectors that are designed to readily separate in the event of contact with a moving vehicle. The connectors are also arranged to minimize exposure of high voltages in the event of separation. In these aspects, the invention will be seen to provide major improvements.
Multi Electric Mfg., Inc., the assignee of the present invention, has developed and received FAA approval for a PAPI Style B system utilizing three 105 watt pre-focused lamps per LHA as opposed to the previous standard of two or three 200 watt lamps. The system includes bypass circuitry to permit continued LHA operation with one or more lamp failures. The total power consumption for each LHA is reduced from 400/600 watts to 315 watts. The current system uses standard tilt switches including pendulum types where contact is made via a ball of mercury. These mercury vial switches are not only environmentally objectionable, they are becoming more difficult to obtain. In Multi's view, an optical pendulum switch is preferred except that it also requires a power connection for a light-emitting diode (LED) and a signal connection for a photo transistor detector, which undesirably adds to power consumption and the number of interconnections. In the Multi system, the lamps are powered through a single isolation transformer requiring two wires entering the LHA. A single 300 watt transformer of minimum 90% efficiency is used, as opposed to the three-100 watt transformers of minimum 85% efficiency in the prior art.
The present invention utilizes the above-described FAA approved Multi Electric PAPI Style B system, with the power for a microcontroller controlled bypass system and an opto-coupler pendulum tilt system being developed from a small power supply energized by the series constant current loop in each LHA. The small power supply in each LHA may be of standard design and still produce an energy savings over the 30 watt transformer used in the master LHA of the prior art for the control circuitry. However, also disclosed herein is a novel power supply that requires only a 2 watt transformer in each LHA.
In prior art systems, the interconnect wiring for the power leads and tilt switch control circuits could have as many as ten connections at the master LHA and eight connections at each of the other (slave) LHAs. In contrast, in the inventive system all of the LHAs are identical, each having only four connections—two power connections and two control connections. There is no master LHA. Further in the inventive system each LHA includes a microcontroller, the power for which (as well as that required to operate the bypass circuits and opto-coupler tilt system) is derived from the constant current lamp circuit. In this context, the term microcontroller connotes an internal memory, as opposed to the usual microprocessor that uses an external memory. The arrangement reduces system cost and complexity and enhances its manufacture, installation and service.